[alma-config] 14 km configuration

[Mark Holdaway]

There was some concern here in Tucson when Bob Brown announced
that the largest ALMA configuration was 14 km, whereas the systems
group had been working with a 10 km assumption.  Bob Brown
stated that when you look at what fits on the site, it turns
out to be a 14 km array.

I noticed in ALMA Memo 296 that the 14 km array comes about
in part because of shadowing due to Chajnantor and Chascon,
and in part because of the donut topology of the configuration.

If we relaxed the shadowing criteria, antennas could fit between
Chajnantor and Chascon.  I didn't see any mention in Memo 296 what the
shadowing elevation angle is.  A lot of the shadowing occurs in E-W
observations, and there is no compelling reason not to relax the E-W
shadowing (ie, make it 20 or 30 degrees elevation in E-W; no objects on
the sky are made inaccessibe by that change, just the HA tracks are
somewhat restricted).  If we relax the shadowing, the inner
antennas in the donut could go closer to Chascon.  Additionally, the
increased freedom in antenna placement would likely result in
a superior configuration and beam.  In evaluating full track
(u,v) coverage, we would have to consider shadowing by the mountains.

Anyway, it seems to me that a smaller (ie, 10km or 12km) array
could easily be fit onto Chajnantor in that case.

Scientifically, there may be a case for 14 km.  However, as the jump
to 14 km was made for technical reasons rather than scientific ones,
I would seek to fit something that was a more respectable jump from
the 3km configuration (such as 10 km).


	-Mark